Implantable devices, particularly mass transfer devices, breast implants, prosthesis and tissue expanders, are generally known. Many of these devices include a sack-like or envelope-like bladder which is manufactured from an elastomeric material such as silicone rubber and then filled with a liquid or gel. The liquid or gel filling can be a silicone fluid or a saline solution. A combination of an elastomeric envelope and liquid or gel fill material is utilized to imitate properties of the tissue being replaced or supplemented in breast implants. An expander may be an expandable bladder with a local or remote port to add or remove liquid or gel. In tissue expanders, the same general design allows variable degrees of expansion.
Initially developed soft pliable prosthetic implants included a smooth surface. It was felt that the smooth surface would elicit a minimal foreign body reaction. This was found to be true, however, the smooth surface prevented attachment of the scar capsule to the prosthesis so that any movement of the host created a shearing effect which lead to a later and perhaps more severe encapsulation response if the prosthesis was subject to trauma or some sort of infection in the area of implant.
Picha, in U.S. Pat. No. 5,002,572, discloses that it is known that the implantation of an article or material into soft tissue initiates a sequence of physiological events in which the body attempts to remove or isolate the foreign entity. The presence of an implant may lead to the formation of a collagen layer of increased density as part of the host's attempt to isolate the foreign body. Such layers are referred to as the "fibrous capsule" and its formation is dependent upon a multiplicity of factors including surgical procedure, implant shape and size, relative movement between and surrounding tissue, and surface morphology of the implant or texture.
Picha discloses an implant in which the surface of the device in tissue contact is molded to provide a regular pattern of micropillars at least 100 microns (.mu.m) in height with transverse dimensions and interpillar spacing each no greater than 5000 microns. Picha discloses that micropillars have been found to influence the density, vascularity, and cellularity of the capsule surrounding the implant.
Ersek et al., in U.S. Pat. No. 4,955,909, disclose that capsular contracture causing firmness of soft silicone implants is a serious problem that may result from several causative factors that have, as their final pathway, the development of increased scar tissue. Ersek et al. state that this problem may be reduced by forming a net-like three-dimensional grid on the silicone surface in order to gain fibroblast ingrowth into the interstices and thus prevent micromotion at the host prosthesis interface.
G. Picha and D. Siedlak, in "Ion-Beam Microtexturing of Biomaterials", MD and DI, pp. 39-42 (April, 1984), disclose a method to manufacture uniform pillar projections on the surface of an implant using ion-beam thruster technology. Although this method overcomes the disadvantages of the open cell method (discussed below), it is believed a costly process and uneconomical to apply to custom-shaped surfaces.
The pillar surface structure of Picha '572, Ersek et al. '909 and Picha and Siedlak ion-beamed surfacing technology is generally depicted in FIG. 4.
In U.S. Pat. No. 4,889,744, issued to Quaid, a method for making a medical implant with an open cell textured surface is disclosed. The implant has an open cell texture produced by applying soluble particles (e.g., salt, sugar, etc.) to an uncured layer of silicone dispersion. The silicone layer is then fully cured. Subsequent to curing, the silicone layer is then placed in a suitable solvent so that the solid particles are dissolved from the surface of the shell. This method creates open cells on the surface of the implantable body. This prior art device is depicted in FIG. 5.
The open cell structure manufacturing technique is believed to pose three potential problems. First, introduction of a foreign or non-silicone particle to the surface of the uncured silicone can affect the properties of the silicone during the curing process or over the life of the implant. The open cell structure also creates potential silicone fragments which can easily become detached from the open cell structure or cell wall as can be readily seen by the physical shape of the cells in FIG. 5. Finally, use of a soluble particle requires that the particle be fully dissolved prior to implant. If the particle is not fully dissolved or the particle becomes encapsulated by the silicone, such particles may be released from the surface after implantation. This may be detrimental.
Thus, it is generally known to alter the surface morphology or topology of an implantable device to improve the host prosthesis interface. It will also be appreciated that the selection of an altered surface configuration requires balancing between having sufficient texturing to provide adequate anchoring of the implant and having a textured design which minimizes adverse body reaction. Thus, the ideal implant provides sufficient anchoring while minimizing the encapsulation response and subsequent scar tissue.
Other considerations in selecting the technology for altering or texturing the implantable device include the cost of the method, along with any potential hazards the method may create in the final implantable device. As was seen with the open cell textured method, the potential for contamination from the soluble particles or fragmentation of a cell wall may pose a problem. Further, the ion-beam thruster method may be considered uneconomical.
Accordingly, a need exists for an improved or enhanced implant surface morphology or topology which provides sufficient anchoring at the host prosthesis interface while minimizing the reaction of the human body to the foreign body implant. Further, a method to manufacture such enhanced surface is needed. The selected implant enhanced surface should also have a closed cell structure without interstices which is economically manufactured. The present invention addresses these needs as well as other problems associated with existing textured surface implants. The present invention also offers further advantages over the prior art and solves other problems associated therewith.